Do you mean you want to determine what the copy number of your plasmid is per bacterial culture, transfected cell, or just the copy number of a purified plasmid?

Generally, the copy number of a purified plasmid is calculated from taking an OD measurement and determining concentration (sorry, I don't have the formula in front of me, but it involves Avogadro's Number.)

Once you know the #copies/ul of your purified standard, you create a standard curve ranging from ~ 10 copies/reaction to 10^7 copies/reaction, then you can determine copy number of your unknown samples. (Be sure to toss out your non-linear points.) If you are trying to measure directly from bacteria or cells, you can probably just add a few dilutions directly to your PCR reactions.